This invention relates to a toner powder density control for a photocopier device, and more particularly to such a control which maintains the toner powder density in the magnetic brush toning unit by replenishment in accordance with the amount of toning required by the latent image being copied.
In photocopier machines electrostatic charge patterns are produced corresponding to the image or light pattern of a subject which is desired to be copied. The image is developed by applying a powder or toner thereto which adheres to the image areas and is fixed thereon. The application of the toner powder requires the use of a carrier such as iron particles which triboelectrically hold the flocculent toner powder so that it may be brought into contact with the electrostatic charge pattern, separated from the carrier, and selectively deposited on the recording surface in accordance with charge. Rotating magnetic development brushes carry the toner coated metallic carrier particles into contact with the recording medium, and the toner is selectively attracted to the image in accordance with the charge thereon. As successive electrostatic charge patterns are developed and copies made, the toner is gradually depleted from the mixture requiring replenishment in order to obtain the proper ratio of toner to carrier, otherwise there will be a gradual change in the developed or toned images producing copies which are nonuniform, improperly shaded and thus diminish the reproduction quality of the copies.
A variety of techniques have been employed in an effort to overcome this problem. One such method optically senses the reflectivity of the toner powder in the development unit to determine whether replenishment is required. However, dust from the process collects on the optical sensor which often causes malfunctions of the unit leading to inadequate replenishment, carrier particle carry-out from the developer unit, and weak copies. Another method involves sampling noncharged areas which pass the brush and activating the replenisher when the potential of the noncharged areas drops below a predetermined value which produces undesirable variable effects on the potential when measured over charged areas. Another method measures the resistivity of the mix using a sensing means immersed in the mix which would be imprecise when carrier is lost and further would depend on the position of the probe as well as the agitation applied to the mix to maintain uniformity therein.
Other methods have similar problems.